I've had similar experiences with bottling when I didn't adequately mix in my priming sugar solution. Some bottles got more sugar (and fruit flavor concentrate, for that particular wheat beer), and gushed. Others didn't. Were you diligent about stirring the beer gently to mix in the sugar?

+1 what RensBerserker said. Not mixing your priming sugar well enough is typically the cause of random gushers.

Also, you said you only filled them to the base of the neck. Are you saying it's like this:
If so part of the problem may be too much head space in the bottle. I tried filling a bottle once about 3/4 full (it was the last bottle and there wasn't enough to fill it but seemed lie too much to pour down the drain) and it gushed on me. So if that is what you're doing, you need to fill your bottles more.

I'm going to lean towards some bottles got more sugar than others, or that some bottles may have been infected. I thought I had thoroughly mixed my sugar, but its highly possible that I did not. I also did not detect any difference in taste among the spewing bottles, but it could have still been infected.

So upon priming, I let the priming sugar in hot water (I always warm up my priming sugar), but this time I forgot about the priming sugar warming up in a sauce pan, came to a boil, and started becoming syrupy. So I thought it would not matter and added it to the beer before bottling. 13 days later, I shared the beer with friends...and some of them erupted violently! So, the priming sugar was not distributed well. My conclusion, never bottle with priming sugar that turns into syrup if left too long in heating.